Single element radiometric lens

ABSTRACT

The disclosure describes systems and apparatuses that include a focusable lens, as well as methods for focusing the optical lens. The focusable lens system includes a single element lens having a concave refractive surface characterized by a first radius of curvature and a convex refractive surface characterized by a second radius of curvature larger than the first radius of curvature. A detector element generates electrical signals representative of infrared rays refracted by the single element lens and incident on the detector element, and an aperture stop is disposed around an optical axis of the optical system and secured in a constant position relative to the detector element, the aperture stop configured to limit a cone angle of rays refracted by the single element lens. They system also includes image processing circuitry configured to generate digital pixilation data based on electrical signals generated by the detector element.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/785,091, filed Mar. 14, 2013, and entitled “Single ElementRadiometric Lens” the disclosure of which is hereby incorporated byreference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

Thermographic cameras and imaging devices detect the spectrum and powerof infrared radiation and use this information to form images. Insteadof the 450-750 nanometer range of the visible light camera, infraredcameras may operate in wavelengths as long as 14,000 nm (14 μm).

Infrared energy is a part of the electromagnetic spectrum andencompasses radiation from gamma rays, x-rays, ultra violet, a thinregion of visible light, infrared, terahertz waves, microwaves, andradio waves. These various categories of infrared radiation are relatedand differentiated in the length of their waves (wavelength). Allobjects emit a certain amount of infrared radiation, which changes as afunction of object temperature.

In general, objects with higher temperatures emit more infraredradiation as black-body radiation. Infrared imaging systems detect thisradiation much like an ordinary camera detects visible light. Infraredimaging systems have been used in various applications, particularlythose operated in low light environments, such as those found atnighttime, in smoke-filled buildings, or underground. Infrared imaginghas been valuable for military, rescue, and wildlife observations.

Despite the progress made in infrared imaging systems, there is a needin the art for improved methods and systems related to these systems.

SUMMARY OF THE INVENTION

The present invention relates generally to infrared imaging devices.More specifically, the present invention relates to single elementradiometric lenses. The present invention is applicable to a variety ofoptical imaging systems.

According to an embodiment of the present invention, a focusable opticalsystem having a single element, radiometric lens is provided. The singleelement, radiometric lens includes a concave refractive surfacecharacterized by a first radius of curvature and a convex refractivesurface characterized by a second radius of curvature, a detectorelement configured to generate electrical signals representative ofinfrared rays refracted by the lens and incident on the detectorelement, an aperture stop disposed around an optical axis of the opticalsystem and secured in a constant position relative to the detectorelement, the aperture stop configured to limit a cone angle of raysrefracted by the lens, and image processing circuitry configured togenerate digital pixilation data based on electrical signals generatedby the detector element.

The lens of the optical system may be configured to be focused by beingdisplaced axially with respect to the optical axis of the opticalsystem, and the aperture stop may be secured such that distance betweenthe aperture stop and the detector element is not altered by focusing ofthe lens. The detector may be configured to be operable over awavelength range of 8 μm to 14 μm, and an f-number of the optical systemmay be between 1 and 2.

According to another embodiment of the present invention, a method ofoperating a focusable optical system is provided. The method includesfocusing the optical system on a first scene located a first distancefrom the optical system. Focusing the optical system on the first sceneincludes axially displacing a single element radiometric lens along anoptical axis of the optical system. Axially displacing the singleelement radiometric lens includes moving a portion of the single elementradiometric lens through an opening in an aperture stop. The method alsoincludes focusing the optical system on a second scene located a seconddistance from the optical system. The second distance is greater thanthe first distance. Focusing the optical system on the second sceneincludes axially displacing the single element radiometric lens alongthe optical axis such that a portion of the single element radiometriclens is moved towards a detector within the optical system and is movedthrough the opening in the aperture stop.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, the present invention provides asingle element radiometric lens. The single element design canfacilitate high image quality, while minimizing the system complexity.The simplified design and manufacturing process can facilitate costsavings as compared with more complex, multi-lens designs. Furthermore,in view of specifications such as the f-number, field-of-view, anddetector size that can be attained through incorporation of aspects ofthis disclosure, this single lens can be made quite small.

The small lens size facilitates use of the lens within a smaller camerabody having a compact design. The small lens size also results inmanufacturing efficiencies associated with diamond point turning,grinding and polishing, or molding during fabrication of the lens. Theseand other embodiments of the invention along with many of its advantagesand features are described in more detail in conjunction with the textbelow and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized illustration of an example infrared imagingsystem including a single element radiometric lens and aperture stopconfigured as described in this disclosure.

FIG. 2 is a generalized illustration of an example infrared imagingsystem being focused on a near scene.

FIG. 3 is a high level schematic diagram illustrating an imageprocessing subsystem for processing image data generated by an infraredimaging system.

FIG. 4 is a flow diagram depicting operations for focusing the infraredimaging system described herein.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present disclosure relates to an infrared imaging system, such as athermographic camera, having a focusable, single element, radiometriclens and an aperture stop. This disclosure describes and illustratesseveral example embodiments of this system. However, the describedembodiments are exemplary and provided for explanatory purposes only.The scope of this disclosure is intended to cover any other such opticalsystems, elements, or components, the design of which could be inferredfrom, or informed by, the information and descriptions presented in thisdisclosure.

During focusing of the infrared imaging system, the lens can be movedback and forth along the optical axis, while the aperture stop anddetector are disposed in fixed positions relative to each other. In thisway, the brightness of the image is unchanged by movement of the lens todifferent positions over the focusing range.

The thermographic camera lens may be a single element f/1.5 germaniumlens providing a 40°×30° field-of-view for a 320×240 detector with a17-micron pixel pitch. However the concepts that will be describedherein can be easily and practically applied to systems that exhibitother specifications, and any such system shall be understood as beingwithin the contemplated scope of this disclosure for all purposes.

The focusable radiometric lens comprises an optical element fabricatedfrom a single piece of refractive material (e.g., germanium, glass,plastic, or the like). The fabrication process may involve machining thelens using diamond machining on a high precision lathe. For this reason,it is preferable for the lens to have a minimum size consistent with theoptical performance characteristics and specifications sought for aparticular infrared imaging system embodying aspects of this disclosure.By minimizing the lens size, the lathing and other processing timesassociated with manufacturing the lens may be reduced, thereby resultingin cost savings.

The fixed position of the aperture stop relative to the sensor resultsin substantially constant illumination being admitted to the image planewhen the infrared imaging system is directed to a constant scene, evenwhile the focus position of the lens is changed. As a result of thefixed position of the aperture stop, the f-number of the infraredimaging system is the same for all focus positions of the lens. Thisperformance characteristic is especially beneficial in thermographybecause of the need for radiometric accuracy throughout all focuspositions.

FIG. 1 is a high level schematic diagram illustrating certain opticalcomponents of an example optical system 160 for infrared imaging. Theoptical system 160 has a single element radiometric lens 110 andaperture stop 140. The optical system 160 is part of a larger overallinfrared imaging system 185. In addition to the optical system 160, theinfrared imaging system 185 includes image processing, memory, interfaceand display elements (not shown in FIG. 1, but specifically depicted inFIG. 3) configured to handle image data generated by the optical system.

In FIG. 1, the optical system 160 is shown being focused on an object105 that emits infrared radiation. Infrared rays 120 radiated fromobject 105 are incident on the radiometric lens 110, which refracts therays towards an image plane (also referred to as a focal plane), where adetector 155 is positioned.

The radiometric lens 110 is a single element lens, and is disposedperpendicularly to the optical axis 115. The optical axis 115 passesthrough the center of curvature of each surface of the radiometric lens110. The radiometric lens 110 may be displaced along the optical axis115 to focus the optical system 160. Focusing in this manner causes thedistance of the lens 110 relative to both the detector 155 and aperturestop 140 to change.

The radiometric lens 110 may be formed of germanium, zinc selenide, zincsulfide, or any one of several other suitable (e.g., moldable) materials(e.g., chalcogenides). The radiometric lens 110 includes a first andsecond refractive surface, as depicted at 125 and 130, respectively. Asbetween the first and second refractive surface 125, 130, the firstrefractive surface 125 of the lens 110 is located furthest from thedetector 155. The first refractive surface 125 is concave and ischaracterized by a first radius of curvature. In an embodiment, thedimensions of the first refractive surface 125 measured in thetransverse plane can be slightly oversized to account for manufacturingtolerances (e.g., ±0.0006″ given a ±0.001″ sag tolerance).

The second refractive surface 130 of the radiometric lens 110 is convex.The second refractive surface 130 may be characterized by a secondradius of curvature and a vertex 135 which is closer to the detector 155than all other points on the lens. Both the first and second refractivesurface 125, 130 may be spherical, aspherical or diffractive.

Bundles of rays 120 pass through the lens 110 and travel in thedirection of the detector 155. However, prior to reaching the detector155, some of the rays 120 at the periphery of each ray bundle areblocked by the aperture stop 140 and never reach the detector 155. Inthis way, the aperture stop 140 serves to limit the cone angle of therays that come to focus at the image plane.

The aperture stop 140 is rounded, and is disposed in a planeperpendicular to the optical axis 115. The aperture stop 140 is rigidlyheld in a fixed position with respect to the detector 155. Hereinafter,the plane in which the aperture stop 140 is disposed will be referred toas the aperture plane (not specifically enumerated). The aperture stop140 has a round opening 140(b) through which admitted rays pass throughprior to reaching the image plane. For example, the opening may have a0.358 inch diameter, represented by the measurement at 140(a).

As described previously, the lens 110 is disposed so as to bedisplaceable along the optical axis of the optical system 160, and maybe moved towards and away from the detector 155 to bring an image intofocus. The optical system 160 is designed such that, at a range ofpositions near the optical infinity focusing position, the lens 110protrudes through the opening 140(b) of the aperture stop 140, causing aportion of the lens 110 to be on the detector 155 side of the apertureplane. FIG. 1 depicts one such disposition of the lens 110 within thisfocusing range. In the depicted disposition of the lens 110, a portion145 of the second refractive surface 135 is in the aperture plane, witha small gap separating it from the aperture stop 140.

As the lens 110 is progressively moved towards the detector 155, therebydecreasing the distance from the lens to the detector and focusing theinfrared imaging system 185 at a greater distance, the portion of thelens 110 located on the detector 155 side of the aperture planeincreases. Simultaneously, at the aperture plane, the gap between thesecond refractive surface 130 and the inner edges of aperture stop 140decreases. At the extreme of the focusing range, when the infraredimaging system 185 is focused at infinity, a small gap remains betweenthe aperture stop 140 and the portion 145 of the second refractivesurface 130 in the aperture plane. The dimensions of the optical system160 components may be selected so that the portion 145 of the lens 110located on the detector 155 side of the aperture plane is 0.014 incheswide, as measured from the vertex 135 along the optical axis 115.

Rays 120 that are admitted through the aperture stop 140 pass through awindow 150 on their way to the detector 155. The window 150 may beconstructed of germanium, silicon or any other material suitable for thespecific operating environment and performance characteristics intendedfor the infrared imaging system 185. The aperture stop 140 can becoupled to the window 150 so as to ensure that these components do notmove relative to one another.

After rays 120 pass through the window 150, the rays pass through ahermetically sealed passage (not shown) forward of the detector 155. Thedetector 155 may be an array sensor (e.g., a focal plane array operatingin the infrared band), bolometer or any other sensor suitable forconverting infrared rays to an electrical signal representative of theray intensity. The detector 155 operates in a vacuum environment in someimplementations, with the window serving as a transmissive element of ahermetically sealed package.

The curvature and thickness 100 of the lens 110 and the aperturediameter (distance across opening 140(b)) of the aperture stop 140 areoptimized so as to enable a narrowest lens capable of generating a sharpimage of objects 105 throughout the focusing range. An f number of f/1.5has been shown to be a preferred parameter for orienting thisoptimization. The focal length of the lens 110 can be set relative tothe aperture stop 140 dimensions so as to achieve an f-number that isappropriate for the specific operating environment of the infraredimaging system 185.

Because the lens 110 is radiometric, as the object 105 moves relative tothe infrared imaging system 185 (e.g., from infinity to 200 millimetersaway), the image focus is adjusted by displacing the lens 110 along theoptical axis 115, towards or away from the detector 155. As this occurs,the fixed attachment of the aperture stop 140 and the detector 155results in the separation between these elements being constant. Thus,provided that the radiation pattern of the object 105 remains constant,a steady amount of infrared radiation impinges on detector 155 and theimage brightness of the object will be unaffected by its movement.

Additionally, the curvature of the second refractive surface 135 may beset so that total internal reflection is avoided at the lens periphery,and the admitted rays fill the aperture plane. By filling the apertureplane with admitted rays, imaging errors may be avoided.

FIG. 1 also depicts example dimensions of components in the infraredimaging system 185, as well as system-wide dimensions, which have provento enable positive infrared imaging performance to be achieved with asmall lens 110. For example, the total length of the infrared system 185may be 20.45 millimeters (mm) with a lens diameter of 12.6 mm or 0.496inches, focal length of 7.47 mm or 0.294 inches, and weight of 3.87grams. The f-number can be 1.5. The operating wavelength range of thelens is in the long wave-infrared region of 8 μm to 14 μm, or midwave-infrared region of 3 μm to 5 μm, in some embodiments.

FIG. 2 is a high level illustration of the infrared imaging system 185being focused on a nearer scene than in FIG. 1. Components depicted inFIG. 2 that are the same as the components depicted in FIG. 1 areenumerated accordingly. Because example functionality, design andstructure of the components was described previously, this informationwill not be further mentioned in the discussion of FIG. 2.

Consistent with the near scene focus position of lens 110 in FIG. 2, thelens 110 is depicted as not protruding through the opening 140(b)provided by aperture stop 140. The entire lens 110 is depicted to theleft of the aperture stop 140 and the aperture plane.

Although the infrared imaging system 185 is depicted as being focused ona near scene in FIG. 2, the radius of curvature of the second refractivesurface 130 is such that rays that are refracted by the lens 110 fillthe opening 140(b) provided by aperture stop 140. This phenomenon, whichis also depicted in FIG. 1, is achieved by using the largest outerrefractive surface 130 radius of curvature for which total internalreflection is avoided at the periphery of the lens 110.

FIG. 3 is a high level schematic diagram illustrating image processingcomponents used to provide image processing functionality within theinfrared imaging system 185. Although FIG. 3 illustrates various exampleimage processing components that may be used within the infrared imagingsystem 185, this drawing is not intended to represent any particulararchitecture or manner of interconnecting the components, as suchdetails are not germane to the techniques described herein. The imageprocessing components may be disposed within a personal computer (PC),workstation, tablet, smartphone or other computing environment suitablefor processing data generated by an optical system 160.

As depicted in FIG. 3, the image processing components include a systembus 502 which is coupled to a microprocessor 503, a Read-Only Memory(ROM) 507, a volatile Random Access Memory (RAM) 505, as well as othernonvolatile memory 506. In the illustrated embodiment, microprocessor503 is coupled to cache memory 504. System bus 502 can be adapted tointerconnect these various components and also to connect components503, 507, 505 and 506 to a display controller and display device 508 andperipheral interfaces such as input/output (“I/O”) devices 510. The I/Odevices 510 may be components such as keyboards, modems, networkinterfaces, printers, scanners, video cameras, or other devices suitablefor interfacing with image processing components. Typically, I/O devices510 are coupled to the system bus 502 through I/O controllers 509.

In one embodiment the I/O controller 509 includes a Universal Serial Bus(“USB”) adapter for controlling USB peripherals or other type of busadapter.

RAM 505 can be implemented as dynamic RAM (“DRAM”) which requires powercontinually in order to refresh or maintain the data in the memory. Theother nonvolatile memory 506 can be a magnetic hard drive, magneticoptical drive, optical drive, DVD RAM, flash memory, or other type ofmemory system that maintains data after power is removed from thesystem. While FIG. 3 shows that nonvolatile memory 506 as a local devicecoupled with the rest of the components in the data processing system,it will be appreciated by skilled artisans that the described techniquesmay use a nonvolatile memory remote from the infrared imaging system185, such as a network storage device coupled with the data processingsystem through a network interface such as a modem or Ethernet interface(not shown).

FIG. 4 is a block diagram illustrating example operations for operatingthe infrared imaging system 185. As depicted at 402 in FIG. 4, theoptical system is focused on a first scene located a first distance fromthe optical system. As depicted at 404, focusing includes displacing asingle element radiometric lens axially, along an optical axis of theoptical system. Axially displacing the single element radiometric lensincludes moving a portion of the single element radiometric lens throughan opening in an aperture stop. At 406, the optical system is focused ona second scene located a second distance from the optical system. Thesecond distance is greater than the first distance. The focusingdescribed at 406 includes displacing the single element radiometric lensaxially along the optical axis, as shown at 408. The lateral movement ofthe radiometric lens is such that a portion of the single elementradiometric lens is moved towards a detector within the optical system,and is moved through the opening in the aperture stop.

With these embodiments in mind, it will be apparent from thisdescription that certain components of the described infrared imagingsystem may be embodied, at least in part, in software, hardware,firmware, or any combination thereof. It should also be understood thatany of these components may interface with various computer-implementedfunctions involving data stored in a data processing system.

Additionally, certain techniques may be carried out in a computer orother data processing system by executing sequences of instructionsstored in memory. In various embodiments, hardwired circuitry may beused independently, or in combination with software instructions, toimplement these techniques. For instance, the described functionalitymay be performed by specific hardware components containing hardwiredlogic for performing operations, or by any combination of customhardware components and programmed computer components. The techniquesdescribed herein are not limited to any specific combination of hardwarecircuitry and software.

Embodiments of the systems described herein may also be in the form ofcomputer code stored on a computer-readable medium. Computer-readablemedia can also be adapted to store computer instructions, which whenexecuted by a computer or other data processing system, such as dataprocessing system 500, are adapted to cause the system to performoperations according to the techniques described herein.Computer-readable media can include any mechanism that storesinformation in a form accessible by a data processing device such as acomputer, network device, tablet, smartphone, or any device havingsimilar functionality. Examples of computer-readable media include anytype of tangible article of manufacture capable of storing informationthereon such as a hard drive, floppy disk, DVD, CD-ROM, magnetic-opticaldisk, ROM, RAM, EPROM, EEPROM, flash memory and equivalents thereto, amagnetic or optical card, or any type of media suitable for storingelectronic data. Computer-readable media can also be distributed over anetwork-coupled computer system, which can be stored or executed in adistributed fashion.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A focusable optical system comprising: a singleelement lens having a concave refractive surface characterized by afirst radius of curvature and a convex refractive surface characterizedby a second radius of curvature; a detector element configured togenerate electrical signals representative of infrared rays refracted bythe single element lens and incident on the detector element, whereinthe single element lens is characterized by: a far scene focus positiona first distance from the detector element; and a near scene focusposition a second distance from the detector element, the seconddistance being greater than the first distance; an aperture stopdisposed around an optical axis of the optical system and secured in aconstant position relative to the detector element, the aperture stopbeing configured to limit a cone angle of rays refracted by the singleelement lens; and image processing circuitry configured to generatedigital pixilation data based on electrical signals generated by thedetector element.
 2. The focusable optical system of claim 1, whereinthe far scene focus position and the near scene focus position aredisplaced axially with respect to the detector.
 3. The focusable opticalsystem of claim 2, wherein focusing of the single element lens does notalter a distance between the aperture stop and the detector element. 4.The focusable optical system of claim 3, wherein the detector isoperable over a wavelength range of 8 to 14 μm.
 5. The focusable opticalsystem of claim 1, wherein an f-number of the focusable optical systemis between 1 and
 2. 6. The focusable optical system of claim 1, whereinthe single element lens comprises germanium.
 7. The focusable opticalsystem of claim 1, wherein the single element lens is configured to bemoved axially throughout a focusing range, and wherein the focusingrange is such that, at multiple focusing positions in the focusingrange, the lens protrudes through the aperture stop and a separationdistance between a portion of the lens and the detector element is lessthan a separation distance between the aperture stop and the detectorelement.
 8. A method of operating a focusable optical system, the methodcomprising: focusing the optical system on a first scene located a firstdistance from the optical system, wherein focusing the optical system onthe first scene includes axially displacing a single element radiometriclens along an optical axis of the optical system, wherein axiallydisplacing the single element radiometric lens includes moving a portionof the single element radiometric lens through an opening in an aperturestop; and focusing the optical system on a second scene located a seconddistance from the optical system, wherein the second distance is greaterthan the first distance, wherein focusing the optical system on thesecond scene includes axially displacing the single element radiometriclens along the optical axis such that a portion of the single elementradiometric lens: is moved towards a detector within the optical system;and is moved through the opening in the aperture stop.
 9. The method ofclaim 8, wherein focusing the optical system on a first scene is suchthat, after being displaced axially, all of the single elementradiometric lens is located to a first side of the aperture stop,wherein the detector is disposed on a second side of the aperture stop,the first side opposite the second side.
 10. The method of claim 8,wherein axially displacing the single element radiometric lens alters adistance between the aperture stop and the single element radiometriclens.
 11. The method of claim 10, wherein axially displacing the singleelement radiometric lens alters a distance between the single elementradiometric lens and the detector.
 12. The method of claim 8, whereinthe detector is operable over a wavelength range of 8 to 14 μm.
 13. Themethod of claim 8, wherein an f-number of the focusable optical systemis between 1 and
 2. 14. The method of claim 8, wherein the singleelement lens comprises germanium.